Swappin’ And Droppin’

Widening the Range of Rearends to Swap

One of the few drawbacks when building a ’60s-era GM truck is the limited amount of choices when it comes to updating the rear axle assembly. Since the majority of these trucks came from the factory with manual transmissions, they usually are found to have a rather deep rear gear ratio. Now if you’re a stoplight racer or use your truck to haul or tow heavy loads, then a low gear ratio is generally a good thing. Those of us who like to drive our hotrods at highway speed desire a higher ratio to lower the engine RPM. This not only saves wear on your engine, but usually results in better fuel mileage.

Many truck owners change the front suspension from the earlier six-lug to the later ’70s style five-lug bolt-pattern disc-brake setup. Although there are six-lug front-brake rotors available in the aftermarket, many rodders choose to swap out the rear axle assembly to a later five-lug model.

Starting in 1960 and running until the 1972 model year, GM utilized a trailing arm/coil spring rear suspension design on the Chevrolet trucks, and a rear leaf spring arrangement on the GMC models. Either brand could be special ordered from the factory with the opposite suspension, which explains why you’ll see a Chevy with rear leafs and a GMC with rear coils.

Your hub for horsepower
Get first access to hit shows like Roadkill and Dirt Every Day

For those truck owners with a coil rear suspension, it’s possible to install virtually any rear axle assembly under your truck by changing the mounting saddles. The pre-’70 rearends are 1 1/2 inches narrower than the later ’70-87 units.

But as long as a rearend with 3-inch axle tubes (take a look at Ford 9-inch as well) is used, it can be swapped in without much hassle. A new bolt pattern will require new wheels so a change in track width is not a problem as it can be made up in wheel offset.

Replacement axle saddles are available from Early Classic Enterprises that can be welded to a donor rearend at the correct angle and position. The only other dilemma has been the lack of a mounting provision for a Panhard bar on the replacement housing. Early Classic solves this problem with their Super Track Bar kit, which replaces the original Panhard bar design.

Their kit is designed to mount in the original driver’s side frame bracket, but instead of the opposite end attaching to the axle housing, it runs all the way over to the passenger side trailing arm. The 12-inch longer bar design reduces the radial arc, which lessens the side to side movement of the rearend as the suspension moves up and down.

Follow along as we show the installation of a later model rearend into a ’70 Suburban, along with an ECE 5-inch rear drop kit to change from the original six-lug to newer five-lug bolt pattern wheels.